In vertical MOSFETs used for, e.g., switches of power sources, it has been requested to improve breakdown voltages while reducing on resistances. In a high-voltage vertical MOSFET according to the related art, the parasitic resistances of a body contact layer and a well layer are reduced by increasing the dopant concentrations of the body contact layer and the well layer and the potential of a channel region is stabilized, so that the breakdown voltage is improved.
Referring to FIG. 9, the following will describe the structure of the high-voltage vertical MOSFET according to the related art.
FIG. 9 is a sectional view of the structure of the high-voltage vertical MOSFET according to the related art, showing the structure of a pair of transistors.
As shown in FIG. 9, in the high-voltage vertical MOSFET according to the related art, an N-type drain layer 31 is formed on an N-type semiconductor substrate 30 by an epitaxial method and a P-type well layer 32 including a channel region is formed in contact with the drain layer 31 on the front side of the N-type semiconductor substrate 30. Further, multiple N-type source layers 33 are selectively formed on the well layer 32 on the front side of the N-type semiconductor substrate 30. On the source layers 33, trenches 35 are formed that are internally covered with gate insulating films 34 and reach the drain layer 31 from the well layer 32. Formed in the trenches 35 are gate electrodes 36. On the surface of the well layer 32 between the source layers 33, a recessed structure 37 is formed in contact with the two adjacent source layers 33. Further, a body contact layer 38 is formed immediately under the bottom of the recessed structure 37. Moreover, insulating films 39 are stacked over the gate electrodes 36. Furthermore, a source electrode 40 is formed on the source layers 33, the body contact layer 38, and the insulating films 39. When a voltage is applied to the source layer 33, the same voltage is applied to the body contact layer 38. On the back side of the semiconductor substrate 30, a drain electrode 41 is formed.
In such a semiconductor device, the body contact layer 38 is formed in the bottom of the recessed structure 37 by implanting a group IIIb dopant such as boron from the front side of the semiconductor substrate 30, reducing connection resistance between the source electrode 40 and the well layer 32. For example, the body contact layer 38 is formed with a dopant concentration of 5E20/cm3 by implanting 5E20/cm3 of boron.
In this configuration, the application of a bias voltage to the gate electrodes 36 electrically connects the source layers 33 and the drain layer 31. At this point in time, a source voltage is applied to the body contact layer 38, so that the potential of the well layer 32 serving as the channel region is stabilized and the breakdown voltage is improved.